This invention relates to a process for the manufacture of through-hole contacted flexible circuit boards which are suited for high bending stresses and the circuit boards derived therefrom.
Flexible circuit elements are commonly used in electronic components that are subjected to high dynamic bending stresses, such as, for example, in disk storages and printers. These flexible circuit elements must have a smooth and extremely clean surface, in order to avoid disturbances of the electromechanical operation (of the disc drive or other electronic component) as the result of particles released from the flexible circuit board; particularly particles released from the adhesive materials used in manufacturing the flexible circuitry.
Circuit boards for high bending stresses which have two circuit layers with through-hole contacts located outside the bending area, are usually designed to be flexible on one side and to have carrier and covering films of equal thickness (symmetrical laminar construction). The base material used is a carrier film, made, for example, of polymide, with copper-laminated on both sides thereof. Generally, the copper layer or foil is attached to the carrier film by means of an adhesive layer. The formation of through-hole contacts and a conductive pattern (on the copper foil) is accomplished in a conventional manner by a substractive process (etching). As a result, in the bending area, the conducting paths (circuit pattern) are formed on only one side thereof, and the other side is completely etched away. The conductors are then provided with a protective covering film, of the same thickness as the carrier film.
Despite the apparent symmetrical construction of the above-described flexible circuits, the conducting paths thereon are not located in the transverse section of the laminate that remains "neutral" when subjected to bending stress, that is, the section that is neither subjected to tensile or compressive strains (the .music-flat.neutral section"). In fact, the adhesive layer of the carrier film (which has been etched away) will produce an asymmetry, which acts to displace the neutral section into other transverse areas. In addition, as a result of the exposed adhesive, the circuit board is liable to accumulate dust and to be subjected to abrasion and thereby contaminate the electronic component.
U.S. Pat. No. 4,626,462 (all of the contents of which are fully incorporated herein by reference) attempts to solve the above-discussed problems by a through-hole contacted flexible circuit board for high bending stresses and method of manufacture thereof. In U.S. Pat. No. 4,626,462, a process is provided wherein through-hole contacted flexible circuit boards for high bending stresses can be manufactured with a strictly symmetrical laminar construction in the bending area. In that area, the conducting paths (circuit pattern) are located in the "neutral section" described above. Significantly, there are no exposed layers of adhesive in the bending area which could act as a contaminate.
U.S. Pat. No. 4,626,462 starts with a standard flex circuit material composed of copper foil adhesively secured to a polymeric film and then adds a second copper layer by selectively adhering copper foil to the polymeric film. The adhesive is used in only non-flexing regions of the circuit. A characteristic of the manufacturing technique is that during the process, there is an air gap (identified as 9 in FIG. 2 of U.S. Pat. No. 4,626,462) formed between the polymeric film and the second copper layer in the area containing the dynamic flexing section of the final product.
While the manufacturing process of U.S. Pat. No. 4,626,462 is suitable for its intended purposes, there is a need for improved methods of manufacturing multilayer circuits with dynamic flexing regions.